Existing Routing Protocols

In this chapter, we introduce several routing protocols in WSNs, which can be categorized into flat routing, hierarchical routing, location based routing, and source routing. Flat routing is that when a node queries for data in its communication range, neighbor nodes which have data will transmit the data to that node which is data-centric routing, such as SPIN [4] and Direct Diffusion [11]. Hierarchical routing builds a hierarchical topology in the region. It performs data aggregation and fusion in cluster heads in order to decrease the number of transmitted messages to the base station, such as TTDD [5], LEACH [12], and PEGASIS [13]. As to the location based routing, sensor nodes are addressed by means of their locations. We can use the direction of the destination to forward data to the nearest neighborhood recursively until reaching the destination, such as GEAR [6] and SPAN [14]. Source routing is a routing technique in which the source node determines the complete sequence of nodes to forward the packet;

the source node explicitly lists this route in the packet’s header, identifying each forwarding “hop” by the address or node’s ID of the next node to transmit the packet on its way to the destination node, such as DSR [7], AODV [15], and SPEED [16]. In the following, we review a classical routing protocol in each category.

3.1.1 Flat routing – Sensor Protocols for Information via Negotiation (SPIN) [4]

SPIN, also known as Sensor Protocols for Information via Negotiation, disseminates all the information at each node to neighbor nodes in the network. A source node first advertises its data to neighbor nodes. If a node within the radio range wants the data advertised by the source node, it sends a reply packet to the source node.

6

-The source node will send data to the request node which queried for the data. This is illustrated in Figure 1. The disadvantage of SPIN is that it can’t guarantee the delivery of interested data. If a node is interested in the data that is far away from the source node and nodes between source and destination nodes are not interested in that data, data will not be delivered to the destination node at all.

B B

Figure 1. Flat routing – SPIN. Node A starts by advertising its data to node B (a). Node B responds by sending a request to node A (b). After receiving the requested data (c), node B then sends out advertisements to its neighbors (d), who in turn send requests back to B (e,f) [4]

3.1.2 Hierarchical routing – Two Tier Data Dissemination (TTDD) [5]

An approach that provides data delivery to multiple mobile sinks is called TTDD, is illustrated in Figure 2, which is two tier data dissemination. Each data source proactively builds a grid structure that is used to disseminate data to the mobile sinks by assuming that sensor nodes are stationary and location aware. The sink chosen by the server may leave its old position to another position. Once this occurs, sensor nodes

7

-surrounding the sink process the signal sent by the sink and one of them becomes the new source node to generate data reports.

B

S

Node Source Sink

Figure 2. Hierarchical routing – TTDD. One source B and one sink S [5]

3.1.3 Location based routing – Geographic and energy aware routing (GEAR)

This algorithm, as illustrated in Figure 3, discusses the use of geographic inform

[6]

ation while disseminating queries to appropriate regions since data queries often include geographic attributes. Each node in GEAR keeps an estimated cost and a learning cost of reaching the destination through its neighbors. The estimated cost is a combination of residual energy and distance to the destination. The learning cost is refinement of the estimate cost that accounts for routing around holes in the network. A hole occurs when a node does not have any closer neighbor to the target region than itself. Two phases are used in this algorithm, forwarding packets towards the target region and forwarding packets within the region. In the first phase, when receiving a packet, a node checks its neighbors to see if one neighbor node is closer to the target region than itself. In the second phase, if the packet reaches the target region, it can be

8

-diffused in that region by using recursive geographic forwarding or restricted flooding.

Node

Data packet

re 3. Location based routing – GEAR. Recursive geogra

Target region

Figu ic forwarding: data disseminate to four

SR) [7] and ESDSR [2]

he to store

aintenance operation is the mechanism which a node is able to detect any

ph sub-regions recursively until regions with only one node are left [6].

3.1.4 Source Routing – Dynamic Source Routing (D

The DSR routing [7] is based on the source routing. It uses each node’s cac information about the routing path which will be maintained by each sensor node if the routing path is broken or not. Two operations are used to build a routing path, route discovery and routing maintenance. Route discovery is the mechanism by which a source node finds a route to the destination. When a source wants to send information to the destination, it searches its own cache to find a routing path. If the source node can not find a routing path to the destination, it starts to perform route discovery. First of all, the source node initiates a local broadcast to start the routing. If the node which receives the packet is the route destination, it will route back to the source node. If it is not the destination, it also broadcasts a packet to its neighbors until reaching the destination.

The source will choose the shortest path — the smallest hop count path — to route to the destination.

The route m

changes in the network topology. While a node is unavailable, its one hop neighbor nodes around the routing path should route back to the source such that every node’s caches can update the new information.

9

-ESDSR [2] is an energy efficient DSR protocol. It changes the way of choosing a routi

g Protocols

A ty, position awareness, and

loca

ng path from the shortest path in DSR to the maximum expected life of the path.

Once a source node setups a route discovery, the return packet of each route path will contain the minimum expected battery life. The source node chooses a routing path with the maximum expected battery life among all routing paths. Each node also has its power table which contains its neighbor’s power state information so that it will choose more energy efficient path to the destination.